1. Field of the Invention
The present invention relates to sensors for measuring a parameter of a variable physical structure. The variable structure may include a fluid within a vessel, or it may include a movable solid target. More specifically, the present invention relates to sensors that measure the parameter through the use of an electromagnetic field that is disposed in a volume that includes the variable structure. In the case of a fluid variable structure, the parameter is measured and utilized to determine a fluid level. In the case of a solid target, the parameter is measured and utilized to determine a position, velocity, or acceleration of the target as the target moves along a sensing axis.
2. Description of the Prior Art
According to the present invention, a fluid level can be measured through the application of an electromagnetic field, without the use of a float. The present invention also teaches the measurement of a movable target without physical contact between the sensor and the target. The simple use of an electromagnetic field for measuring liquid level and position is old in the art. Liquid level sensors using an electromagnetic field and having no float (floatless) have been produced with advantages over those that utilize a float. These advantages include lower cost, and removal of the possibility a float becoming stuck in one position so that it does not follow the liquid level.
Position sensors using an electromagnetic field for non-contact measurement have been produced using capacitive, inductive, and eddy current technologies.
Matulek, U.S. Pat. No. 6,164,132, teaches a dual capacitive sensor array liquid level indicator. An array of capacitive sensors is superposed on each side of a planar sensing element, one sensor from each side working together as a pair. The arrays are connected to electronic circuits that determine which sensor pairs are submerged in the liquid and which are not submerged. This provides a liquid level measurement with a resolution depending on the number of sensor pairs in the array. A detection circuit is associated with each sensor pair. Disadvantages of this sensing system, compared to the present invention, include limited resolution, higher electronics cost, and limited sensitivity.
In Netzer, U.S. Pat. No. 6,490,920, a compensated capacitive liquid level sensor is taught with at least three electrodes forming at least two capacitances that vary with the liquid level. The capacitance versus level functions of the two capacitances differ from each other. A ratio of the functions is utilized for compensation of liquids having various permittivities. Disadvantages of this sensing system, compared to the present invention, include a very small signal level, resulting in reduced stability when the liquid level is low, and the requirement for a more complicated set of electronics. When the liquid level is very low, both measured capacitances are very small, and are therefore easily affected by parasitic capacitances and electrical noise. Since the two capacitances are ratioed to compensate for the liquid permittivity, the stability of the result is reduced.
Pchelnikov and Nyce, U.S. Pat. No. 6,293,142 B1, teach a liquid level sensor apparatus in which an electrodynamic element produces an electromagnetic field, in the form of at least one slowed electromagnetic wave (slowed-wave), within a volume that contains the measured liquid. An electromagnetic field parameter is measured that varies with the measurand. The electromagnetic field has a suitable distribution for measuring of the variation in propagation constant of the slowed electromagnetic wave as the electromagnetic field parameter.
In contrast to the prior art inventions cited above, the present invention provides a floatless fluid level sensor with low cost, high resolution, and good sensitivity. When used for fluid level sensing, the present invention provides an advantage of distributed impedance over the length of the sensing element, while operating in a desired frequency range with increased resolution. The measurement of impedance, rather than propagation constant or resonant frequency, allows the use of simple and inexpensive circuitry. Distributing the impedance along the length of the sensor enhances the ability of the sensing element to control the shape of the electromagnetic field, while still operating in a relatively low frequency range for a given sensing element length. The lower frequency range allows the use of lower cost electronics. The shaped field allows measurement of the desired target while largely ignoring other nearby conductive and dielectric materials. The increased resolution allows the design of higher performance sensors.
When used as a linear position sensor, the present invention provides the advantages of non-contact measurement, while being producible at low cost, and with an easily adjustable measuring length. Here also, the measurement of impedance, rather than propagation constant or resonant frequency, allows the use of simple and inexpensive circuitry. As in a fluid level sensor, distributing the impedance along the length of the sensing element enhances the ability of the sensor to control the shape of the electromagnetic field, while still operating in a relatively low frequency range for a given sensing element length. Again, the lower frequency range allows the use of lower cost electronics. This is in contrast to prior art non-contact position sensors, for example:
In Fiori, U.S. Pat. No. 4,637,265, a non-contact sensor apparatus uses the combination of a stationary coil and a movable coil, connected into stationary and movable tank circuits, which are inductively coupled to produce a double resonance curve in the stationary tank circuit. Disadvantages of this sensing system, compared to the present invention, include the need for an electronic circuit disposed within the movable member of a position sensor, and the need for a substantially more complicated set of electronics as taught in the specification. Another disadvantage of this system is the need for a stationary tank circuit, meaning that once fabricated, the sensor length can not be changed without destroying the tank circuit.
In Brosh and Fiori, U.S. Pat. No. 4,658,153, a non-contact sensor apparatus comprises a fixed planar board with serpentine coil structures of relatively low resistance and inductance, driven in resonance mode with a frequency output according to the position of a movable planar member. Some disadvantages of this system include the need for highly stable electronics, since the inductance is low, and the fixed sensing length, which cannot be easily changed once the sensing element has been fabricated.